The present invention relates in general to the use of ultrasonics to inspect liquids in containers and, in particular, to a method and apparatus for ultrasonically inspecting liquids in containers to determine the condition of the liquids wherein the ultrasonic waves are produced by electromagnetic acoustic transducers (EMATs).
The use of ultrasonics to inspect liquids inside containers using conventional ultrasonic testing methods is well established in the art. In such testing, a piezoelectric (or similar) transducer is coupled to the wall of the container using some form of coupling media such a liquid. The ultrasonic sound waves then propagate through the wall of the container and into the liquid inside the container. The sound wave may then reflect from a solid object in the liquid, or a liquid-air interface, or the opposite wall of the container, and be detected by the ultrasonic transducer. In other applications, a pitch-catch arrangement of two transducers on opposite sides of the container is used to launch and detect the ultrasound. A wide range of measurements and liquids using ultrasonics is possible. Some of the possible measurements include liquid height, detection and imaging of solid objects in liquids (for example medical ultrasonic imaging of internal organs), velocity measurements of liquid flow, and attenuation measurements to determine the condition of the liquid.
Nagata et al. (U.S. Pat. No. 4,821,573) discloses a method and apparatus for ultrasonically inspecting the food contents of a package. An ultrasonic transmitter-receiver system is disposed on at least one side of the package and the occurrence or degree of degradation of the contents is determined based upon the output data from the system. The invention is stated to be particularly useful to inspect foods, pharmaceutical agents, feedstuffs and so-on, and they may be of any desired consistency but only if it is freely-flowable, such as a homogenous solution, a dispersion, a paste, or the like. Before the package is subjected to the ultrasonic inspection, it is preferably shaken so as to disperse the headspace (plenum within the packaging material) into the contents. However, depending upon the types of contents, the shaking process may be omitted. This system may be disposed either in contact with the exterior surface of the package or a short distance therefrom and may be disposed on the exterior wall surface of a water tank when the packages are subjected to ultrasonic inspection while immersed in water. Transmitters can be disposed on only one side of the package with the ultrasonic receiver on the other side, or the ultrasonic transmitter and receiver can both be disposed on the same side of the package so that the receiver receives a reflected ultrasonic wave. By measuring differences in sonic velocities between a transmission wave and a reception wave or the sonic velocity of a reception wave, the time from transmission to reception, and/or the degree of attenuation of the ultrasonic energy, the occurrence and degree of degradation of the contents can be evaluated. The useful wavelength of the ultrasonic waves disclosed is about 0.5 MHz to about 20 MHz. While only a single frequency can be employed at a given time, the accuracy of the evaluation is said to be improved by using a few selected frequencies for the transmission wave. Various sets of tests results based on measurements of reflected waves, transmitted waves, and the like are provided to show how various signals can be related to the condition of the food product contents.
As indicated in the various references cited on the face of Nagata et al., sonic or ultrasonic assessment of the condition of foodstuffs has been around for some time.
Clark et al. (U.S. Pat. No. 2,277,037) discloses a fruit ripeness tester which measures the degree of ripeness of fruit such as melons and pineapples by the measurement and calibration of the vibration characteristics of such objects.
Martner et al. (U.S. Pat. No. 3,357,556) discloses a method and apparatus for testing canned liquid material without removing the material from the can, and is particularly suited for the inspection of batch-prepared infant formula. The system is used to detect alterations in viscosity distribution such as by formation of curds or semi-solid bodies or the like, or increases in viscosity as a liquid material ages which is referred to as "age-thickening". The cans containing the liquid are rolled along a horizontal path at a preselected constant speed and in the path is disposed a narrow barrier having a height that is small with respect to the diameter of the can. By proper adjustment of the height of the barrier, cans containing a liquid material that is satisfactory will pass over the barrier whereas cans containing spoiled or aged-thickened contents will be arrested by the barrier. The "slushing" or flow pattern of the canned liquid material within the cans is responsible for cans "bouncing" off of the barrier and rolling backwardly.
Baird (U.S. Pat. No. 3,553,636) discloses a non-contacting ultrasonic interface viscosity and percent solid detecting device wherein the transducer is mounted out of contact with the processed liquid. Changes in ultrasonic attenuation characteristics are used as indications of changes in viscosity, percent solids, and/or interface level condition of the liquid contained within a vessel.
Kreula et al. (U.S. Pat. No. 3,913,383) discloses a method and apparatus for testing the contents of packages containing liquid product. Packages of interest are sealed is and contained liquid food products have physical properties that can change as a consequence of deterioration of the product. The package to be tested is placed on a movable support which is subjected to a sudden movement of short duration. A characteristic dependent on the movement of the support is detected, and signals are generated in response to the detected characteristics and compared with the preselected reference signals. In essence, the hydrodynamic behavior of the contents of the package are used to determine whether or not the contents have changed or spoiled.
Edwards (U.S. Pat. No. 4,208,915) discloses a method for determining foreign material in food products using ultrasonic sound. A plurality of transducers are disposed in a rotatable cylinder having a liquid couplant. The cylinder has a surrounding flexible wall which is compressed on top of the surface of the food products. The sound frequencies are transmitted through the food products and received back by a receiver in the transducers for monitoring any variance in the frequency which indicates foreign material in the food products. Black et al. (U.S. Pat. No. 4,384,476) discloses a method and apparatus for ultrasonically inspecting foodstuffs in which the fluid is passed through a curtain of ultrasonic sound. Reflection or absorption of the ultrasonic sound by extraneous materials is detected by ultrasonic sound receiving means and appropriate indication of such detection is given. The foodstuffs being inspected, however, are inspected as they flow pass the inspection point, and have not yet been packaged.
Jarman et al. (U.S. Pat. No. 5,372,042) discloses an ultrasonic inspection method for determining the seal integrity of the bond lines in sealed containers. These particular packages have a lid bonded to a container rim and it is the seal between the lid and the container that is to be inspected. The container rim is disposed between an ultrasonic transmitter system and an ultrasonic receiver system for inspection.
Wertz et al. (U.S. Pat. No. 5,167,157) discloses an ultrasonic method and apparatus for inspecting laminated products, particularly to determine the thickness of the innermost layers of the article. The mean of the measurements from each transducer placed on either side of the multilayered article is calculated to determine these thicknesses. The articles themselves are laminated plastic articles.
Hayward et al. (U.S. Pat. Nos. 3,832,885 and 3,802,252). Hayward et al. '885 discloses a method and apparatus for inspecting sealed containers, such as vacuum-packed cans of food, which involves repeatedly energizing electromagnetic transducer coils mounted in close proximity to the containers to cause the enclosures of the containers to vibrate at a frequency that is the function of the internal pressures within the container. Sounds produced by the vibrating enclosures provides a tonal pattern distinctive of the presence or absence of a container with an unsatisfactory internal pressure. Changes in the internal pressure are indicative of leakage or food spoilage or corrosion of the container. Hayward et al. '252 is drawn to an apparatus and method for monitoring the pressure or vacuum in a sealed container which consists of striking the can with a magnetic pulse of force to cause it to vibrate freely and thereby generate an acoustic ping sensed by an electrical pick-up device. The frequency of the ping is a function of the internal pressure in the container and the frequency spectrum of the signal output of the pick-up device is examined at a discriminator circuit to determine if the signal output contains selected frequencies at an energy level indicative of the desired pressure level. If not, the container is rejected. Measures are taken to render the signal output of the pick-up device insensitive to both the ambient noise and the large noise pulse generated while the can is being subjected to the magnetic pulse of force and also so that only the purest part of the signal generated in response to the acoustic ping is examined by the discriminator circuit.
A liquid or gel couplant is required for conventional ultrasonic testing of liquids in containers, which represents an additional cost. However, conventional ultrasonics using liquid or gel couplants are not practical at high temperatures. Additionally, in some cases, the surface of a container to be inspected may be contaminated with hazardous materials which would cause the couplant used in conventional ultrasonic testing to become contaminated requiring it to be treated as hazardous material for clean up and disposal. Finally, some metal or partially metal containers have an outer layer of non-conductive material such as paint, wrappers, labels, lids, or coating that would prevent the coupling of ultrasound into the container from a conventional ultrasonic transducer.
Electromagnetic acoustic transducers (EMATs) are sensors which are capable of launching and receiving sound waves in metals without a coupling media or even without contact with the surface of a workpiece being inspected. The ultrasound is launched and received in the surface of the metal by the interaction of magnet fields and eddy currents generated by the EMAT. EMATs are finding a wide range of nondestructive testing applications for metals. One previous application of EMATs to liquid measurements is known. In this case an EMAT was used to launch two types of plate wave modes in the metallic wall of a vessel containing a liquid. One mode was the N=1 Shear Horizontal (SH1) mode. The other mode was the N=0 Symmetric (SO) Lamb wave mode. The SH1 mode propagates in the wall of the vessel, producing only shearing displacements at the liquid-metal interface inside the vessel. Because a liquid cannot support a shear force, the SH1 mode is not attenuated by the liquid as it propagates. The SO mode produces substantial displacements normal to the surface of the metal at the metal-liquid interface as it propagates. Because a liquid does support compressional waves which are generated by displacements normal to the surface of the metal at the metal-liquid interface, the SO mode is attenuated by the liquid as it propagates. By placing a transmitter EMAT and a receiver EMAT a fixed distance apart vertically on the wall of the vessel, and measuring the relative amplitudes of the two modes, an indication of the liquid level between the two transducers can be obtained.
As previously noted, certain types of liquid containers have an outer layer that prevents inspection of the liquid contents therein using traditional ultrasonic techniques. Some containers are provided with outer plastic lids, caps or the like which leave an air gap between the outside surface of the container and the lid sealing the liquid contents. Traditional ultrasonic inspection techniques requiring physical coupling between the transducer and the workpiece being inspected cannot inspect through such air gaps. Some types of containers have thin metal walls or at least one thin metal foil wall.
It is thus apparent that an improved technique for the ultrasonic inspection of liquids in these types of containers is needed and would be welcomed by the industry.